1. Field of the Invention
The present invention pertains to nuclear magnetic resonance apparatus for scanning and imaging, and more particularly to nuclear magnetic resonance apparatus which includes a primary magnetic field produced by a permanent magnet configuration, formed of a combination of relatively low energy flux magnetic material and relatively high energy flux magnetic material.
2. Description of the Prior Art
Nuclear magnetic resonance (NMR) apparatus has been employed only relatively recently for scanning and imaging of biological tissue as an aid in the practice of medicine. In 1974, U.S. Pat. No. 3,789,832, issued to the inventor of the present invention, for use of NMR scanning and imaging for medical applications. The primary components of NMR apparatus presently known for imaging of biological tissue, and more particularly whole body imaging of live biological tissue, include a primary magnetic field, an rf field, and a detector for detecting a nuclear magnetic resonance signal. The tissue to be imaged is subjected to the primary magnetic field and simultaneously an rf field of a particular frequency. Reradiated energy is detected by a detector and the signal therefrom is processed as desired to provide imaging.
In order to effectively image living biological tissue, a magnet must be constructed of sufficient size to accommodate the human form therebetween. In addition, the primary magnetic field generated by the magnet must be of sufficient strength and also must provide a substantially flat field over a relatively large area in order to be useful for its intended purpose.
NMR apparatus for such applications employ air core solenoids or resistive air core electromagnets. In order to obtain stronger magnetic fields a shift was later made to air core super conducting solenoids. These superconducting solenoids or electromagnets must be maintained by a vacuum system at approximately 1/1,000,000,000 of a vacuum and must be maintained at a temperature of approximately 4.degree. above absolute zero or -269.degree. C. The necessary vacuum systems are expensive, difficult to manufacture and maintain, and are extremely vulnerable. Cooling is accomplished by a combination of liquid helium and liquid nitrogen. These substances boil off in a vacuum and constantly must be replenished. Costs of $100,000 a year in liquid helium and nitrogen to keep an air core superconducting solenoid in operation are not uncommon. While vacuum systems and cooling systems have been designed which will provide functional NMR apparatus for use in imaging biological tissue, it is apparent that these devices are largely undesirable because of the precise atmosphere these superconducting electromagnets must be kept in and because of the cost of maintaining the proper atmospheric conditions.
Furthermore, magnetic lines of flux generated by air core solenoids cover an immense area and, since interference with these lines of magnetic flux can cause inappropriate NMR responses, elaborate sanctuaries uninvaded by materials or objects which would affect the magnetic field must be constructed.
Because of the relatively large flat working field of magnetic flux needed and because of the relatively high field strengths needed for NMR imaging of biological tissue, it has been heretofore thought impractical to employ a permanent magnet for such an application. Presently available low energy flux material, such as ceramic magnets, although inexpensive, do not produce sufficient field strength per unit volume so that a magnetic field produced by an aggregation of such magnets must be unreasonably large making this material impractical for use. Therefore, the use of low energy flux magnetic material is largely impractical since such magnets would have to be of an enormous size. The suggestion of using a permanent magnet in an NMR apparatus was made in the present inventor's U.S. Pat. No. 4,354,499. The present invention moves such a construction from the possible to the commercially practical.
High energy magnetic flux material is available for use and sufficient concentrations of magnetic field can be produced when these materials are aggregated but the cost of these materials is prohibitive and therefore their use has been ignored. Use of high energy magnetic flux materials such as rare earth magnets therefore is entirely impractical because of the cost involved in production and assembly of such a magnet.
The present invention overcomes the problems associated with the prior art by providing a permanent magnet configuration for use in nuclear magnetic resonance scanning and imaging apparatus for generation of a primary magnetic field which incorporates a combination of low energy flux magnetic material, high energy flux magnetic material, and flux conductors in a configuration which concentrates the magnetic flux over a region of sufficient area, sufficient uniformity and at a sufficient strength to produce a usable magnetic field for imaging of biological tissue and in particular for imaging of living whole body biological tissue. In addition, the area in which the permanent magnet configuration of the present invention can be employed is reduced, relative to air core type magnets, through magnetic shielding accomplished by placement of magnets of particular polarity at selected areas about the permanent magnet to reduce the magnetic radiation therefrom and to concentrate the magnetic field. Such a combination has not been heretofore known for use in generating a primary magnetic field for nuclear magnetic resonance scanning and imaging.
Generally, the use of a permanent magnet in an apparatus employing NMR principles is shown in U.S. Pat. No. 4,240,439 issued Abe et al on Dec. 23, 1980. The apparatus of Abe teaches a probe for observing NMR phenomenon for small surface areas of a living body or areas disposed therein by transcutaneous insertion of the probe. The use of a permanent magnet configuration which generates a field sufficient to permit whole body scanning is not shown or suggested.
Similarly, the use of permanent magnets in nonbiological NMR apparatus is shown in U.S. Pat. Nos. 2,894,199 issued to Kirchner on July 17, 1959 and 4,350,955 issued to Jackson on Sept. 21, 1982. However, generation of a magnetic field of the type produced by the present invention through the use of a combination of high and low magnetic flux magnets, and flux conductors in combination is not shown or suggested.
Aggregation of magnets is known as shown in U.S. Pat. Nos. 3,059,156 to Moriya on Oct. 16, 1962 and 3,889,220 issued to Spodig on June 10. 1975, however neither of these references shows or suggests a manner for combining magnets in a form suitable for producing a magnetic field of sufficient strength and of sufficient flatness for use in imaging of biological tissue, and more particularly, to whole body imaging.
Furthermore, the shielding of permanent magnets to concentrate the magnetic field generated by a permanent magnet in an NMR device is not shown in the prior art. While teaching for shielding of magnets is shown in U.S. Pat. Nos. 2,398,653 issued to Linlor on Apr. 16, 1946; 2,993,152 issued to L. Pierce on July 18, 1961; 3,768,054 issued to Neugebauer on Oct. 23, 1973; 3,168,686 issued to King on Feb. 2, 1965; and 3,205,415 issued to Seki on Sept. 7, 1965, none of these references show or suggest the use of such shielding for NMR apparatus and none show the use of shielding on a magnet which physically even remotely approaches the size and complexity magnet necessary for whole body NMR imaging.
Maintaining a flat field is important in NMR apparatus. A technique for accomplishing this is shown in U.S. Pat. No. 2,930,966 issued to Bell on Mar. 29, 1960.